A lithium ion secondary battery is often used for a power source of a small electronic device such as a notebook type personal computer, a mobile phone, or a PDA. A lithium ion secondary battery has been further used also for an automobile application in recent years. As a range of use of a lithium ion secondary battery is expanded, a demand for performance and safety of the lithium ion secondary battery (hereinafter, also simply referred to as a battery) is increasing. These batteries are usually used repeatedly by charge and discharge operation. However, due to repeated charge and discharge, a volume change of an electrode and heat generation caused by charge and discharge may increase an internal pressure of the battery, and an electrolytic solution may leak out. In this case, not only deterioration of battery characteristics but also heat generation due to a decrease of an electrolytic solution, corrosion of a device due to the electrolytic solution, and the like occur disadvantageously. For example, in a case where an electrolytic solution is used for a lithium ion secondary battery, the electrolytic solution is an organic solution and extremely dislikes water. Therefore, such a lithium ion secondary battery requires a high sealing property to completely prevent infiltration of water into the battery and to completely prevent leakage of the electrolytic solution.
For example, a lithium ion secondary battery is housed in a metal container in order to seal a power generating element thereof. In order to prevent short circuit between a positive electrode and a negative electrode, it is necessary to insulate a positive electrode terminal from a negative electrode terminal. Generally, a gasket made of an insulating material is used in an opening of a metal container containing a power generating element for insulation and sealing between positive and negative electrodes. As an insulating material, it is known to use a resin insulating gasket (for example, see Patent Literature 1).
In order to further strengthen sealing by such an insulating gasket, it has been proposed to use an insulating gasket and a sealing agent together (for example, see Patent Literatures 2 to 4). By applying this sealing agent to an insulating gasket or a metal container, and attaching the insulating gasket to a metal container, a sealing property between the insulating gasket and the metal container is increased.
Examples of such a sealing agent include a pitch material such as coal tar or asphalt, and a material obtained by adding a polymer as a modifying agent to the pitch material (for example, see Patent Literature 5). In addition to the pitch material, a butyl rubber (for example, see Patent Literature 6), a diene-based rubber having a predetermined weight average molecular weight (for example, see Patent Literature 7), a block polymer containing a diene-based monomer (for example, see Patent Literature 8), and the like have been proposed.
By the way, in recent years, an influence of volatile organic compounds (VOCs) on the environment has become a problem. The above sealing agent is used as a sealing agent composition obtained by dissolving or dispersing the sealing agent in an organic solvent as a VOC. Therefore, a sealing agent composition using no VOC is demanded.
Patent Literature 9 has proposed an aqueous sealing agent composition. However, performance demanded for a sealing agent is not sufficient. For example, strength of a sealing agent layer formed by the aqueous sealing agent composition is insufficient compared with that of a sealing agent layer obtained by a sealing agent composition using an organic solvent. In addition, Patent Literature 10 has proposed an aqueous sealing agent composition using no VOC. However, a battery using an aqueous electrolytic solution is a target, and no consideration has been given to a non-aqueous electrolyte battery.